Which of them could beat the other, one on one?
One on one, we see:
Centipede vs. Toad [DRAW]
-I say draw because the only reason Centipede got taken was because Hybrid interfered. Centipede was getting lots of hits in, but they weren't having any effect. Toad wasn't getting hurt, but he couldn't land a punch on the much faster Centipede. I don't know how this one would have panned out in a strict duel.
Snake vs. Toad [TOAD]
-Snake wasn't having any effect and didn't know the weak spot. and by the end he was taking serious hits. If it hadn't have been for Scorpion's interference, he would have lost.
Scorpion vs. Toad [SCORPION]
-Scorpion knew the weak spot and had the weapon to take it out at a safe distance.
Centipede vs. Lizard [CENTIPEDE]
-Lizard gets a lucky hit in early, but in their subsequent combat Centipede is getting in hit after hit.
Snake vs. Hybrid [SNAKE?]
-Snake, but only because of what the master said about Hybrid being limited. Actually, their duel is inconclusive, with no hits either way.
Snake vs. Scorpion [?]
-Scorpion gets the stinger cheap shot, but Snake gets the hit-to-the-stomach cheap shot. Hard to tell which would win in a clean fight, which they never have.
Hybrid vs. Scoripon [SCORPION]
-After a while, Scorpion gets in a devastating kick.
All in all, I'd say Centipede and Toad are by far the most powerful. In the end, I'd give the edge to Centipede, because while it isn't clear whether Centipede could ever beat Toad, as soon as someone takes out Toad's weak spot, he's done.
Scorpion has a big buildup, but he clearly overrates himself. Most of his damage is done via his weapons. Fighting wise, he only gets one really good hit in, and that's on the mere Hybrid.
Lizard hardly fights one on one at all, but he doesn't seem terribly effective.
Snake is shown to be quite weak compared to the others.
Hybrid is the weakest, but only slightly weaker than Snake.